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Thermal Energy Storage Systems: a backup for air conditioners

By on August 26, 2015

Development in technology has left man basking with luxuries. In the growing world, air conditioning has been a prerequisite. With the proliferation of industries the increase in load on the power grid is alarmingly high. Increasing power cuts in the country due to glaring demand-supply gap, the increases in power tariffs and innovative tariff plans like time of day (TOD) calls for the need to store energy. DG Sets and Inverters/UPS have been in use for all kinds of applications for quite some time, but there was no solution for air conditioning in the past. Thermal energy storage (TES) apparently provides the most prominent solution.

Why Thermal Energy Storage?

Cooling finds its use at home and industries. It is an important aspect for man, to lead a comfortable life. Air conditioning the house during summer, refrigeration to preserve food etc. has been a basic requirement for common man.  On the other hand, the demand for cooling in an industry has increased. For example, a chemical industry needs air conditioning to store chemicals in a stable condition.

Air conditioner works on electricity. The demand for electricity has grown through the years. Subsequently, the load on the power grid has increased drastically. This has put us in a predicament where the demand is not met by the supply from grid which leads to shortage of electricity. Given the situation, Thermal energy storage would be an apt solution to the growing energy demands. It is adopted to balance energy demand during on and off peak conditions. It can also reduce dependence on DG sets and inverters during power cuts.

How utilities manage peak demand using TOD (Time of Day Tariff)

For utilities or electricity distribution companies it is very expensive to buy additional power during peak requirements if the supply does not meet demand. The rate at which electricity is supplied does not make it financially feasible for electricity distribution companies to supply the electricity at the same rate. So utilities apply a new tariff plan called as Time of Day (or TOD) tariff. As per this tariff plan, electricity is costly during peak consumption hours and cheaper during the low consumption period. This kind of tariff plan is applicable in most states for industries and in some states for commercial setup as well. Thus using TES at times when electricity is cheap can help save some costs when electricity is expensive. To know more about TOD tariff read our article: Understand Time of Day (TOD)/Time of Use (TOU) tariff structure and how it impacts customers.

Where is TES used?

  • The TES is used to supply electricity during peak demand. It supplies the peak load whereas the grid continues to supply the base load.
  • The TES can also be used when the installed cooler does not meet the required capacity. The TES works with the installed cooler to supply the system.
  • Sometimes the TES can also be used as a standalone system for cooling.
  • Just as UPS backs a computer, The TES can be used as a backup for the installed cooling system during power cuts.

How does a TES system work?

The thermal energy storage system is a tank that is filled with a storage medium. It has a built up refrigeration system which cools the system. Pipes and tubes connect the TES system with the air conditioning unit. The storage medium is water in most cases. The water is chilled to low temperatures during periods of low demand by direct cooling. The energy is thus stored in the medium till it is discharged. When the demand increases, hot air is allowed to pass through the tank. Thus it absorbs the cold by conduction principle. This cold air is transported back to the air conditioner through connecting pipes or tubes.

Types of TES available in the market

Chilled water storage:

In this case, the sensible heat capacity of water is used to store energy. Energy is stored at 4 to 5 degree Celsius.

Ice Storage:

It uses the principle of latent heat of fusion of water to store energy. Energy is stored at 0 degree Celsius. Some additional fluids called charging fluids are mixed with the water at -5 degree Celsius. Charging fluids act as anti-freeze agents which prevent the water from freezing.

PCM Storage:

In this method, the phase changing materials are contained in nodules. Nodules are hollow spherical structures. They freeze at selected temperatures. This principle is used in TES to cool the system.

Molten Salt Energy Storage

In this type of storage technology molten salt is employed as a storage method. It has a high efficiency of 99% and is used quite often with a Solar Tower to store thermal energy.


Full Storage:

In this mode of operation, The TES shifts the entire load to off peak hours. It operates at full capacity to charge during non peak hours. During peak conditions, the full demand is met by the TES. The chiller doesn’t run.

It is best used when the charges are high during Peak conditions

Partial Storage:

When Chiller capacity is less than load, chiller meets part of the On peak load and the TES takes the rest. Here, the chiller runs at full capacity through the day. When load is lesser than the capacity, TES stores energy. When load is greater than the capacity, TES discharges energy.

It is best used when peak load is highly greater than average load.

Space required:

Given the operation of TES, one would be wondering how much space the system would occupy. You don’t need much space at all. It can be compared with a water heater at home. A typical 1700 sq. ft. home has a 4 sq.ft. water heater. The water heater uses (4 sq. ft. / 1700 sq. ft.) only 0.23% of the floor space. A 70 sq. ft. tank will cool about 10,000 sq.ft. and use about 0.70% of the floor space for full storage. Most thermal energy storage systems are partial storage systems where storage accounts for about 30% of the total cooling required reducing the floor space needed to about ¼ of 1% of the conditioned space.

Maintenance Required:

TES is not a high maintenance device and needs some bare minimum maintenance steps:

  • The coolant concentration must be checked every year. An annual test is conducted to check the level of corrosion inhibitors and the amount of anti freeze agents (if applicable).
  • The cooling water must be treated properly in order to enjoy high efficiency.

Benefits of using TES

  • The TES system uses the Time of Day (TOD) tariff to its advantage. It charges in the night when the tariff is low and discharges in the morning when tariff is high. Thus, it reduces the cost of electricity. Air Conditioning is the biggest component in most commercial electricity bills and TES can help reduce the same.
  • Usage of DG sets and inverters, which are very costly and inefficient, can be avoided by using TES. Moreover the pollution from DG sets can be effaced by using TES
  • With TES bolstering the conventional cooler, the chiller size is reduced. Thus, the cost of setting up a chiller is reduced. In other words, the capacity of cooler is increased for the same cost.
  • The amount of refrigerants used up by a conventional chiller is quite high. This can be reduced by installing TES
  • The TES also reduces the electricity consumption
  • With zero emission, the TES is rendered eco friendly
  • Finally, the TES is highly efficient and reliable
  • Some utilities give benefits on using TES systems in a commercial setup.

Drawbacks of TES:

  • TES systems are expensive to buy. They are mostly used in commercial setups where the rates of electricity are higher and thus payback is earlier. Although they can be used in residential setups as well, but the paybacks are much longer.
  • TES systems are not available for smaller requirements (they are mostly available in sizes of 3 tons and above) and thus can be used only in larger setups.

Here is a short video for tips to buy the best air conditioner in India



About the Author:
Abhishek Jain is an Alumnus of IIT Bombay with almost 10 years of experience in corporate before starting Bijli Bachao in 2012. His passion for solving problems moved him towards Energy Sector and he is keen to learn about customer behavior towards Energy and find ways to influence the same towards Sustainability. .